Flint optical glass

ABSTRACT

A weather-proof, flint, optical glass composition comprising, on a percent by weight basis, from 12.5 to 19.5% SiO2, from 8.0 to 16.5% Al2O3, from 12.0 to less than 20.0% B2O3, and from 42.0 to 60.0% PbO.

United States Patent Kuwayama 1 June 10, 1975 FLINT OPTICAL GLASS [58] Field of Search 106/53, 47 Q [75] Inventor: Shigeo Kuwayama, Odawara, Japan [73] A F Ph t F] C td Mham, [56] References Cited ss1gnee: ll]! 0 0 1 m 0., 1 1- Ashigara, Japan UNITED STATES PATENTS 3,149,984 9/1964 Faulstich 106/53 Flledi g- 21, 1973 3,653,933 4/1972 Tsunekawa 106/53 21 A 1. No.: 390 262 I 1 pp Primary Examiner-Winston A. Douglas Related Apphcatlon Data Assistant ExaminerMark Bell [63] Continuation-impart of Ser. Nos. 182,876, Sept. 22, Attorney, Agent, or Firm-Sughrue, Rothwell, Mion,

1971, abandoned, and Ser. No. 33,500, April 30, Zinn and Macpeak 1970, abandoned, and Ser. No. 681,583, Nov. 8, 1967, abandoned. 7 ABSTRACT [30] Foreign Application priority Data A weather-proof, flint, optical glass composition com- Nov 9 1966 Ja an 4l 73651 prising, on a percent by weight basis, from 12.5 to p 19.5% SiO from 8.0 to 16.5% A1 0 from 12.0 to 52 us. 01. 106/53 less than 20% B208 and from to 600% Int. Cl C03c 3/04; C03c 3/10; C03c 3/30 10 Claims, No Drawings .FLINT OPTICAL GLASS CROSS-REFERENCE TO PREVIOUS APPLICATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to flint optical glass having excellent weather-proof properties and a high refractive index n 2. Description of the Prior Art In the design of lenses, it is desirable to eliminate the so-called secondary spectrum. For such a purpose, two types of optical systems have been offered. One system uses a compound lens in which the ratio of the refractive index to the Abbe number, n/v, differs from each other. This type of correction for both chromatic and spherical aberration is based on the phenomena that curved aberration in the image plane is small and spherical aberration can be minimized. The optical system under such achromatic conditions is called apochromat," which is suitable for use as objectives in a microscope or telescope, and condensing lenses (see US. Pat. No. 3,653,933). The glass composition disclosed in US. Pat. No. 3,653,933 is a four component glass comprising PbOB O SiO Al O in which a relatively high level of SiO is utilized. While the presence of SiO in a large amount provides desirable characteristic in designing an achromatic lens, this high percentage renders the glass unstable, i.c., the glass tends to be milky.

The other system is the so-called anastigmat using a compound lens in which the separate components compensate for the astigmatism of each. In other words, since a combination of lenses having substantially the same ratio of refractive index to Abbe number (rz/v) is selected, curved aberration in the image plane can be reduced while spherical aberration is large, thereby obtaining a compound lens corrected for astigmatism. This type of lens system is suitable for adaptor lenses, etc. From the practical standpoint, it is thus desirable that flint glasses and crown glasses used in the same lens system have, at best, the same partial dispersive power. However, flint glass tends to have a considerably higher partial dispersive power to blue light than crown glass. The partial dispersive power to blue light of the crown glass is generally about 0.55, e.g., crown glasses having a partial dispersive power of 0.544 and 0.551 are commercially available.

The ordinary flint glasses available on the market have the partial dispersive power of e.g., 0.605 and 0.608. Up to the present time, there have been found several species of speci'al-flint-glas'ses having substantially the same-refractive'inde'x ri and dispersive power 11,, as the ordinary flint glasse's and having about 0.01 lower partial dispersive pow er to'blue light. For example, one of the special flint glasses exhibits such excellent properties that the partial dispersive power of blue, 6, is 0.559, this flint glass being of a lead-aluminaborate type, whereas, in an ordinary flint glass of n However, the above-mentioned special flint glass has the disadvantage that the weather-proof properties are very bad due to the borate contained therein. In some cases, the flint glass is weathered even during polishing, so it cannot be used unless sandwiched with other lenses. In the case of making a high grade lens having little aberration, in particular, this is an important limitation.

In addition, a flint glass having a high refractive index has been desired to meet various types of optical design requirements.

SUMMARY OF THE INVENTION The inventors have made efforts to improve the weatherproof properties of the flint glass and found that a glass of the following composition has good characteristics:

B 0 12.0 less than 20.0 percent by weight A1 0 8.0 16.5 percent by weight SiO 12.5 19.5 percent by weight PbO 42.0 60.0 percent by weight in which the upper limit of B 0 is hereafter designated as being in the range of less than 20.0 percent by weight. The refractive index n of the flint glass ranges from about 1.68 to about 1.80.

DETAILED DESCRIPTION OF THE INVENTION The glasses of the present invention may be prepared, for example, by melting a composition containing boric acid (H BO aluminum hydrate Al(OH) minium (Pb O,) and flint (SiO in proportions such that a glass having the required proportions of these ingredients, as set forth above, may be obtained.

The composition is then melted in a platinumor platinum alloy-lined vessel at a temperature of l,l00 1,450C. for 5 to 10 hours followed by stirring, cooling, casting and annealing. Either an inert or oxidizing atmosphere may be employed. The presence of a reducing atmosphere should be avoided since such an atmosphere increases the tendency of the product to devitrify.

To illustrate the present invention in greater-detail, a flint glass having the following raw material composition was prepared.

Table I Composition (grams) H 30 Boric Acid 229.0 Al(OH Aluminum .0

Hydrate Pb -,O Minium 608.0 AS 0 Arsenious 2.0

Acid SiO Flint 169.0 La Q, Lanthanum l 8.0

Oxide 3 Table II Melting Casting Annealing Cooling Similarly, glasses having different compositions were prepared, the ingredients and proportions of which are 10 also shown in Table III.

TABLE Ill Example Number Composition of Glass (wtf/l) 1 2 3 4 3 0,, 12.9 15.1 17.6 19.5 A1 8.8 9.3 10.0 10.7 PbO 59.4 56.7 53.7 49.7 As. ,O-, 0.2 0.2 0.2 0.8 16.9 16.7 16.4 17.0 La Q, 1.8 2.0 2.1 2.3 ThO ZnO 51,0 Characteristics Measured 1. Specific Gravity 4.48 4 30 4.12 3 92 2. Water Resistance W (mg) 0.10 3. Acid Resistance S Loss in Weight (mg) 75.4 Loss Percentage (71) 1.68 4. Optical Constant n,, 1.73765 1.71936 1.70122 1.68093 n 1.73103 1.71315 1.69547 1.67706 11 F 1.75450 1.73497 1.71573 1.69427 11,, 1.76826 1.74778 1.72752 1.70412 I'lp 11 0.02347 0.02182 0.02026 0.01721 l/D 31.4 33.0 34.6 39.6 5. Partial Dispersive Power of Blue Light 0.586 0.587 0.582 0.572

= n n /n 11 In Table III, the compositions and characteristics of Table IV the glasses of the Examples are shown. In the measurement of the characteristics, the weather-proof property Compis judged by the water resistance and acid resistance, pnem Amoul of Compmem Added whose measurement methods are as follows: Lower than the Within the range Higher than the Lower Limit of the Invention Upper Limit Water Reslstance W slo Weather Resis- 12.5-19.5 wt Tends to Be Miltance Lowers ky A Sample of glass 15 pulverlled, washed wll-h water A1203 Tendency To Be 8.0l6.5 wt Devitrification then with alcohol and dried to give a powdered glass Mllky 2332 CH3 sample of a slze of 0.20 0.30 mm. The resultlng sam- BJ 6 Increases 1104655 than weather Resis ple 15 charged to a 200 ml. Erlenmeyer flask of quartz 20.0 wt tance Lowers glass with 100 ml. of distilled water, allowed to stand Pbo {ffif Be Be for 1 hour on a hot water bath and the amount of alkaline components exuded is titrated with 1/100 N hydrochloric acid using Methyl Red as an indicator, followed by conversion into an equivalent Na O(mg.). The lower this amount, the better the water resisting property.

Acid Resistance S A similarly treated glass sample of a size of 0.50

0.60 mm. (specific gravity (g)) is charged to a cage of verted into a percentage to the specific gravity (g). The

larger this value, the larger the solubility, that is, the lower the acid resistance.

sistance cannot be obtained and other serious disad-' vantages such as the tendency to be milky or devitrification occur. The criticality of the components in the ranges is shown in greater detail in the table below, with reference to comparison examples, wherein the values underlined indicate ranges outside those described for the present invention.

Table V Composition Run No. of Glass ("/1 wt.) A B C D E F SiO 19.5 12.5 17.7 12.5 15.9 17.0 A1. .O 10.0 8.3 16.5 12.2 10.6 8.2 PbO 41.2 64.0 23.5 43.5 49.4 61.8 13. 28.0 15.0 38.5 29.5 20.3 11.0 As. ,O 0.3 0.2 0.3 0.3 1.5 0.2 Lar o 1.0 3.5 2.0 2.3 1.8 Milky Devitril De\'itrifi De- De- Decation at cation at vitri vitrivitri- 800C. 1000C fication fieation fication at 920C. at the as a surface whole As is clearly shown in Table IV and Table V above, the ratio of each ingredient in the flint glass in accordance with the present invention is critical. Referring to B 0 which is the most important factor determining the properties of the flint glass, the objects of the present invention cannot be achieved with compositions outside the ranges called for in the present invention. That is, when B 0 is formulated in a lower amount than the lower limit thereof, not only the partial disperweight) and the loss percentage by weight was 6.9 percent.

The weather resistance of prior art glasses was further examined in greater detail, in which the amounts of B 0 and PbO were slightly modified so as to avoid falling within the criterion set forth in the present invention. The results are shown in Table VI below in which the underlined values indicate those outside the scope of the present invention.

Table VI Comparison Example No.

sive power of blue, 0, increases but also devitrification occurs over the whole of the glass (Run F); when B 0 is contained in a higher amount than the upper limit, the weather resistance lowers and devitrification is caused. Furthermore, a high level of PbO serves in providing an optical flint glass having a high refractive index n Thus, an optical flint glass suitable for optical design can be obtained.

When the water resistance and acid resistance of the ordinary special flint glass were measured, to determine the weather resistance thereof, according to the afore-mentioned method, the water resistance W (conversion to Na O) was 1.98 mg, the acid resistance S showed a 616 mg. loss in weight, and a 19.5 percent loss percentage. In the ordinary optical glass, heavy barium crown glass (SK 16, n 1.620, 11,, 60.3) that had been considered to be inferior in weather resistance, the water resistance W was 2.58

mg., the acid resistance S was 249 mg. (loss in ily weathered.

As is evident from the results of the foregoing examples and comparison examples, the weather resistance of the glass of the invention is far superior to that of ordinary glasses. The partial dispersive power of blue, 0,

of the glass in Example 2 of this invention is larger than those of the other examples, but about 0.01 smaller than that of the ordinary glass having similar optical constants, 71 and 11 (n 1.73608, 11 32.0), that is 6 0.595.

In this invention, moreover, various oxides were added to the glass of the composition of Example 1 in order to examine the influences thereof, thereby obtaining the result that one or more of Ta O La O ThO and Nb O may be jointly used, but the sum thereof must be less than 10 percent by weight in order to retain the desired characteristics of the glass. The amount of Bi O must be less than 10 percent by weight, since amounts of greater than 10 percent by weight result in some increase in 0. For example, at 10 percent by weight, 0.563. The amount of BeO and ZrO must be less than 2 percent by weight, since amounts of greater than that result in devitrification. Sb O may be added up to a proportion of 1 1 percent by weight, although the weather proof property tends to be lowered as the amount of Sb O increases. The amount of P 0 must be less than 1 percent by weight due to its devitrification tendency. ZnO and CdO may be added in a proportion of about percent by weight, but, in adding greater than that, the 11,, rises to n WO and TiO may be added up to about 4 percent by weight, but addition in excess thereof results sometimes in an increases of 0. Alkaline earth metal oxides, such as BaO and CaO may be added up to about 5 percent by weight and, in particular, where 11,, to ri may be higher, more may be added. Furthermore, AS203 may be used as a fining agent in a small amount of up to about 5 percent by weight, and alkali metal oxides may be added, if in a proportion of about 5 percent by weight.

As illustrated above, the glass of the invention gives -a smaller partial dispersive power of blue and a better 8 consisting essentially of on a percent by weight basis from 12.5 to 19.5 percent SiO from 8.0 to 16.5 percent A1 0 from 12.0 to less than 20.0 percent B 0 and from 42.0 to 60.0 percent PbO.

2. The glass of claim 1, wherein the glass contains up to 10 percent by weight based on the total weight of glass of an oxide selected from the group consisting of Ta O La O ThO Nb O and mixtures thereof.

3. The glass of claim 1, wherein the glass contains up to 11 percent by weight based on the total weight of glass of Sb O 4. The glass of claim 1, wherein the glass contains less than 1 percent by weight based on the total weight of glass of P 0 5. The glass of claim 1, wherein the glass contains u to about 10 percent by weight based on the total weight of glass of ZnO.

6. The glass of claim 1, wherein the glass contains up to about 4 percent by weight based on the total weight of glass of an oxide selected from the group consisting of W0 TiO and mixtures thereof.

7. The glass of claim 1, wherein the glass contains up to about 5 percent by weight based on the total weight of glass of alkaline earth metal oxides.

8. The glass of claim 1, wherein the glass contains up to about 5 percent by weight based on the total weight of glass of AS203.

9. The glass of claim 1, wherein the glass contains up to about 5 percent by weight based on the total weight of glass of alkali metal oxides.

10. The glass of claim 1, wherein the refractive index n of said flint optical glass ranges from about 1.68 to about 1.80. 

1. A WEATHER-PROOF FLINT OPTICAL ANASTIGMATIC GLASS CONSISTING ESSENTIALLY OF ON A PERCENT BY WEIGHT BASIS FROM 12.5 TO 19.5 PERCENT SIO2, FROM 8.0 TO 16.5 PERCENT AL2O3, FROM 12.0 TO LESS THAN 20.0 PERCENT B2O3 AND FROM 42.0 TO 60.0 PERCENT PBO.
 2. The glass of claim 1, wherein the glass contains up to 10 percent by weight based on the total weight of glass of an oxide selected from the group consisting of Ta2O5, La2O3, ThO2, Nb2O5 and mixtures thereof.
 3. The glass of claim 1, wherein the glass contains up to 11 percent by weight based on the total weight of glass of Sb2O3.
 4. The glass of claim 1, wherein the glass contains less than 1 percent by weight based on the total weight of glass of P2O5.
 5. The glass of claim 1, wherein the glass contains up to about 10 percent by weight based on the total weight of glass of ZnO.
 6. The glass of claim 1, wherein the glass contains up to about 4 percent by weight based on the total weight of glass of an oxide selected from the group consisting of WO3, TiO2 and mixtures thereof.
 7. The glass of claim 1, wherein the glass contains up to about 5 percent by weight based on the total weight of glass of alkaline earth metal oxides.
 8. The glass of claim 1, wherein the glass contains up to about 5 percent by weight based on the total weight of glass of As2O3.
 9. The glass of claim 1, wherein the glass contains up to about 5 percent by weight based on the total weight of glass of alkali metal oxides.
 10. The glass of claim 1, wherein the refractive index nD of said flint optical glass ranges from about 1.68 to about 1.80. 